Ecosystem studies: Florida Keys

(2007-Ongoing) — The future of Keys biodiversity in the face of a rising tide: Our research program has been involved in the Florida Keys since Mike Ross’ tenure at the National Audubon Society.  Today we are beginning a set of studies which follow up on a paper we published in 1994, “Sea level rise and the reduction in pine forests in the Florida Keys”.  Since that time, the condition of pine forests and other fresh water communities in the lower Keys has become considerably more precarious, especially as a result of inundation by salt water during Hurricanes Georges (1998) and Wilma (2005).  Current studies in collaboration with Dr. Keqi Zhang’s research group from FIU’s International Hurricane Research Center combine storm surge and ground water modeling, floristic information from our fire monitoring plot network [see Fire Ecology Project], and detailed topographic and vegetation structure data derived from LIDAR flights, to project the future of Keys biodiversity in the face of a rising tide and (potentially) more violent storms.  With funding from the US Fish and Wildlife Service, The Nature Conservancy, and the US Department of Energy’s Climate Change Research program, we hope to provide coastal resource managers with a firmer foundation for decisions they will be called upon to make in years to come.

 

(2002)— Soil-productivity relationships and organic matter turnover in dry tropical forests of the Florida Keys:  Florida Keys pine rocklands are supported by thin organic soils developed directly on limestone bedrock. These areas maintain a fine balance between accretionary and degradational processes as soils tend to be shallow and organic turnover rates are extremely rapid. In this study soils and aboveground production in five types of upland forest in the Florida Keys were examined. Throughout the habitat gradient represented by these forest types, the soils were predominantly shallow and organic, forming in place directly on the limestone bedrock. However, the well-drained soils in the most productive broadleaved forests were deep enough to qualify as Histosols (Folists). Soils decreased in electrical conductivity and increased in nutrient content with increasing aboveground production. At 3–12 Mg ha−1 yr−1, production was within the range reported for dry tropical forests. Measured rates of decomposition were moderate or fast, and estimates of the organic C turnover of several soils based on their bomb radiocarbon signature were 100 years or less. In the face of these rapid turnover rates, we attribute the development of organic soils to the absence of mineral residues from weathering of the underlying limestone bedrock. For production to be sustained nutrient cycling must be very efficient, with little leaching beyond the rooting zone. Maintenance of the diverse Keys forests on the most productive soils may depend on maintaining this efficiency in the face of threats such as sea-level rise and habitat fragmentation.

 
(1998-1999) — Developing Ecological Criteria for Prescribed Fire in South Florida Pine Rockland Ecosystems: The pine rocklands of South Florida, characterized by a rich herbaceous flora with many narrowly endemic taxa beneath an overstory of south Florida slash pine (Pinus elliottii var. densa), are found in three areas: the Miami Rock Ridge of southeastern peninsular Florida, the Lower Florida Keys, and slightly elevated portions of the southern Big Cypress National Preserve. Fire is an important element in these ecosystems, since in its absence the pine canopy is likely to be replaced by dense hardwoods, resulting in loss of the characteristic pineland herb flora. Prescribed fire has been used in Florida Keys pine forests since the creation of the National Key Deer Refuge (NKDR), with the primary aim of reducing fuels. Because fire can also be an effective tool in shaping ecological communities, we conducted a 4-year research study which explored a range of fire management options in NKDR. The intent of the study was to provide the Fish and Wildlife Service and other land managers with information regarding when and where to burn in order to perpetuate these unique forests.

 

(1994-1999) — Forest succession in tropical hardwood hammocks of the Florida Keys: effects of direct mortality from Hurricane Andrew: Like most plant communities, the dry broadleaved forests of the Caribbean basin - of which the "hardwood hammocks" of the Florida Keys are one example - function in an environment characterized by several types of periodic disturbance, including fire, deforestation, and hurricanes. Compared to fire disturbance events, the impact of hurricanes on Caribbean hardwood forests is probably more frequent but also more ephemeral, i.e., recovery of forest structure is relatively rapid (Turner et al. 1997). Hurricanes and less intense tropical storms may therefore play an important role in dry forest succession following fire or human disturbance, but probably do not initiate the successional process themselves, except on a small scale. This project was designed to provide a better understanding of this successional process in the upper Florida Keys. A tree species replacement sequence for dry broadleaved forests (tropical hardwood hammocks) was inferred from species' abundances in stands abandoned from agriculture or other anthropogenic acitivities at different times in the past. Stands were sampled soon after Hurricane Andrew, with live and hurricane-killed trees recorded separately, so it was also possible to assess the immediate effect of Hurricane Andrew on stand successional status.

 

(1993-1998) — Hurricane Andrew and Upland Forest Succession in Biscayne National Park: Major hurricanes are catastrophic events from the human perspective, but their effects in many ecosystem types may be ephemeral to the casual observer, and spatially variable. For instance, functional recovery of upland forests from some Caribbaean hurricanes is rapid (e.g., Whigham et al. 1991), though changes in species composition may continue for several decades or more (Crow 1980). By producing large openings in the forest canopy ---  thereby increasing overall structural heterogeneity --- hurricanes and other high-wind events may create some limited opportunities for rapid accession to the forest canopy by seedlings of post-hurricane origin. Species successful at this opportunistic strategy are apt to be the same that dominate the early stages of stand development following more comprehensive disturbances, e.g., fire or anthropogenic landclearing activities.  Elsewhere in the same stand, the hurricane may create less open conditions, where increase in species dominance occurs primarily through crown expansion by large trees that survived the storm, or by growth into the overstory by  pre-storm advance regeneration or subcanopy trees. Species successful at the latter two mechanisms are likely to be most abundant during the mid- and late-successional stages, respectively, following fire or other catastrophic disturbance. In this study we describe the mosaic of post-hurricane forest stand structure and successional status within two upland forests ("hardwood hammocks") in Biscayne National Park four growing seasons after the passage of Hurricane Andrew (August 24, 1992), To do so, we employed relationships derived from observed species' abundances along a chronosequence of time since abandonment from anthropogenic disturbance in nearby Key Largo.

 
(1996) — A Study of the Distribution of Several South Florida Endemic Plants in the Florida Keys:  With many endemic species in the Florida Keys pine rocklands, and fire suppression threatening the pine forests, it is important to determine the need to conserve these native herbaceous species. We therefore conducted research to examine the need to enact conservation efforts under the Federal Endangered Species Act for five plant taxa. The objectives of our study were to:   1. Describe the distribution of each species; 2. Determine whether within- and among-island differences in species distributions were explainable on the basis of easily measured structural variables; 3. Assess the relationship between fire history variables and species' abundances. Prior to the study, the small volume of information known about these plants came from species lists, herbarium collections, anecdotal accounts and few published works. The development of such distribution and abundance data is essential to design management options capable of increasing the abundance of at-risk plants.

 

(1989-1994) — Sea-Level Rise and the Reduction in Pine Forests in the Florida Keys:  Since 1935, and even dates prior, pine rocklands within the Floida Keys have been contracting. Evidence demonstrates that Sugarloaf Key had 88 hectares of pine forest before 1935. The primary objective of this study was to quantify pine forest contraction within the Florida Keys as it relates to sea-level rise. Using aerial photographs we determined that by 1935 Sugarloaf Key pine forests had shrunk to 46 hectares and by 1991 to 30 hectares. During the 70 years prior to 1991 sea-level had risen 15 centimeters. The pine death occurred first at the lowest elevations and the lower areas show encroachment of halophytic species. Groundwater and soil water salinity were the highest where the most rapid pine reduction occurred and the remaining trees experienced the highest physiological stress. If sea-level rise continues, the Florida Keys will experience a decline in both landscape and species diversity as rich pine forest assemblages are replaced by simple mangrove communities.

 

(1988-1992) — Ecological Site Classification of Florida Keys Terrestrial Habitats:  The determination of site-classifications utilizing vegetation, hydrologic and edaphic underpinnings are particularly useful in the mapping of devegetated areas or in projecting the effects of sea-level rise scenarios. We conducted a study that examined site and vegetation factors in 113 Florida Keys locations that had been undisturbed for at least 50 years. Results show that Keys vegetation is arranged along two environmental gradients: an elevational gradient within islands and a geographical gradient associated with the position along the NE-SW trending island chain. Two-way indicator species analysis (TWINSPAN) was used to divide the sample into 14 major groups on the basis of plant species composition. Finally the TWINSPAN classification was modified to recognize 13 Ecological Site Units which were homogenous in important site factors as well as vegetation characteristics. Plant species diversity increased from intertidal to upland site units, while canopy height, basal area, and fine litter production increased both upslope and downslope of the supratidal units.